DCVs are employed within oil and gas industry subsea production control systems (PCSs) to control hydraulic function lines that are in turn used to open and close subsea hydraulic valves on PCS trees and manifolds. Hydraulic valve operation is fundamental to a subsea PCS and much design focus has been placed on ensuring that valve operations can be assured. A subsea PCS is inaccessible once deployed, very expensive to recover and critical to revenue stream (once operational). Therefore, a subsea PCS incorporates telemetry functions to enable DCV and associated hydraulic valve operations to be remotely monitored. PCS telemetry enables problems to be diagnosed and future problems predicted and circumvented so that recovery can be scheduled into maintenance programmes, weather windows and seasonal access windows. Any enhancement of the diagnostic capability of a PCS is therefore important.